This invention relates generally to liquid crystal displays and in particular to such displays provided with a uniform reflective (background for use when the display is to be used under ambient light conditions) which also contain a built-in source of illumination for viewing the display when the ambient light is insufficient.
U.S. Pat. No. 3,748,018 in the name of Howard C. Borden, Jr., discloses a universal transmission reflectance mode liquid crystal display having a reflector 32 as well as a built-in light source 36 arranged such that its light may be directed to the rear of the display package by means of a mirror 48; however Borden's display is not particularly compact and therefore is not adaptable for use in miniaturized consumer products such as wristwatches and portable calculators.
In U.S. Pat. No. 2,646,637 in the name of R. N. Nierenberg et al entitled "Device For Transilluminating Transparencies," there is illustrated an arrangement wherein a plurality of wedges having light piping properties are used to provide a relatively uniform source of illumination for back-lighting a transparency, a back reflector 15 being employed as a light intensifier. By using at least two such wedges, there is a proported improvement in the uniform distribution of the light; however the device disclosed is not particularly suitable for use with liquid crystal displays and furthermore requires at least two separate light sources be provided. It might also be pointed out that the embodiment of FIG. 6, even though provided with a back reflector, would not permit the reflective mode operation of say, a field effect liquid crystal cell provided with polarizers under typical indoor lighting conditions, inasmuch as most of the light passing through the display would not be reflected back towards the observer by the back reflector but would be generally diffused by the highly roughened surfaces 12 and 12a. In a dynamic scattering liquid crystal display, the situation would be even worse, inasmuch as these roughened surfaces would uniformly scatter all of the light reflected back to the observer, thereby greatly reducing the contrast of the display.
Accordingly, one object of the present invention is to provide a liquid crystal display having built-in illumination.
Another object of the present invention is to provide such a display which is sufficiently compact to be utilizable in a wristwatch or other small portable device.
Yet another object of the present invention is to provide a liquid crystal display with built-in illumination in such a manner that the display can also be used with ambient lighting.
Still another object of the present invention is to provide a light pipe for use in a liquid crystal display which is of a configuration that can easily be mass-produced.
A particular object of the present invention is to provide a combination light pipe/reflector for use with a field effect liquid crystal or other display such that said display may be used either with a built-in illumination source or with ambient room-lighting.
Briefly, the invention which accomplishes these and other objects comprises in its preferred embodiment a light pipe formed of plexiglass or other similar optically clear material having a clear, lightly etched top surface, a patterned bottom surface preferably having closely spaced minute convex protrusions, and an enlarged portion at one edge containing a cavity for the insertion of a light bulb. Reflective surfaces covering the major portion of the external surface of said light pipe may direct the bulk of the light emitted from said bulb to the region located between said top and bottom surfaces.
In the particular embodiment described, the enlarged portion of said light pipe has its transverse axis slightly displaced but parallel to the transverse axis of the region defined by said top and bottom surfaces and is provided with additional sloping and notch shaped reflective surfaces in order to cause the light emitted from said bulb to enter said region in a direction more normal to said surfaces.